1. Field of the Invention
This invention relates to load-indicating fasteners.
The invention relates in particular to fasteners, such as bolts or studs, which in use are subject to tensile loading. It is often important to ensure that the fastener is tightened to a predetermined loading. It is known to use the stretching of the fastener which occurs under tensile loading to give an indication when the appropriate loading has been reached.
2. Description of Related Art
European patent EP 0 049 537B describes a load indicating fastener having a shank with an axially extending bore which receives a gauge pin anchored at its inner end to the shank and protruding outwards beyond an end face of the fastener, with an indicating member rotatable on the protruding part of the gauge pin and arranged so that the indicating member is free to rotate when the fastener is unstressed but when the fastener is subject to a predetermined tensile loading is held against rotation by contact with the end face of the fastener.
The gauge pin and indicator member are usually set with an initial gap between the indicator member and the end face of the fastener such that the gap is closed when the fastener is subjected to the predetermined loading and the end face bears against the indicator member sufficiently to prevent its being rotated by hand.
A difficulty which can arise in use of the fastener described in EP 0 049 537B is that to ensure that the fastener is not over-tightened, the operator has to check the indicator member at frequent intervals during tightening. This problem is particularly acute when the fastener is tightened using a power tool. Moreover, after the fastener has been tightened, there is nothing to indicate whether the fastener has been over-stressed.